1. Field of the Invention
The present invention generally relates to semiconductor integrated circuits and methods for image processing, and particularly relates to a semiconductor integrated circuit and a method for enhancing contours in image signals supplied from an image sensor.
2. Description of the Related Art
A CMOS image sensor includes a reception section in which a plurality of photo-diodes are arranged in a matrix array. These photo-diodes each correspond to a pixel of a captured image. Incident light is subjected to photoelectric conversion on a pixel-by-pixel basis, and the electric charge obtained by the photoelectric conversion is accumulated in an electric charge accumulation section for reading of data. Data of each pixel is supplied to a processing circuit such as an image processor for such processing as RGB conversion, contour enhancement, and format conversion, followed by being supplied to a memory or to an image display apparatus or the like.
In order to enhance image contours, conventionally, uniform image enhancement is applied unvaryingly to all the pixels of an image. Namely, differences of image data levels with adjacent pixels are obtained with respect to every pixel of the image, and these differences are enlarged by a predetermined factor so as to enlarge differences of image data levels between adjacent pixels.
A Japanese Patent Application Publication No. 5-22601 discloses a process that avoids an artifact of multiple contours without degrading the performance of contour enhancement. This process obtains a first-order derivation by calculating differentials in a predetermined direction while providing a dead zone in which no processing is performed, and further obtains a second-order derivation in this predetermined direction, followed by performing an averaging process to generate an edge-enhanced signal.
When all the pixels of image data are subjected to image enhancement, however, image noise is enhanced altogether, resulting in undesirable degradation of image quality.
Accordingly, there is a need for a method and a circuit for contour enhancement that can easily generate contour-enhanced images without degrading image quality.